I rage-quit when some people are being annoying over mike or in text, more often over text, I know I can mute them but it's easier just to change server.
If I'm being insulted. Again, easier to change servers.
If I basically come out of spawn point and immediately get killed, this is usually more when there is a sentry at every exit or a bunch of players camping in one spot. That being said, one of my favourite things to do in TF2 is to run up, as a pyro since that's usually what I play, and flame kill the snipers then try to flame kill players when they come out of spawn. I'm a hypocrite but I usually get bored after they kill me and I don't usually do it again.
Or if no one, or barely anyone is playing.
I don't usually rage quit if someone keeps killing me. I don't really mind but if there is a sentry or someone sitting at every exit so I basically have to sit there killing anything that can go past.